M20B25 Rebuild

[digger]

People who know more than me say that 1.5-2mm is the region to avoid as it is the worst for causing possibility of detonation/knock so if you're going to do it aim for 1.0mm (at most 1.25mm ). If the compression is low, fuel is good, the engine tuned well you can still run it but it's not ideal.

[Coyote_ar]

is it possible to machine down the stock B25 pistons, to use them with 135mm rods? with the 84mm crank i mean.

[digger]

is it possible to machine down the stock B25 pistons, to use them with 135mm rods? with the 84mm crank i mean.

doubt it, that's approx 4.5mm that has to come off (84-75)/2 = 4.5mm. not much room for top land and ring groove after you recut reliefs

2 or 3mm should be ok and a thicker gasket so it might be ok with eta crank. you can find some 24V slugs and cut them down with a dome profile similar to the OEM b25 slugs. id link you to a thread but e30tech is down

[Coyote_ar]

the dome is really important for performance in a turbo engine? i could try to get some M52B28 pistons.

i did the math and i need at least 3cc volume in the piston to get the CR low enough.

[digger]

the dome is really important for performance in a turbo engine? i could try to get some M52B28 pistons.

i did the math and i need at least 3cc volume in the piston to get the CR low enough.

thats seems to be debatable, id build it the same way as a NA engine but thats just me (maybe 1.25mm clearance), i dont see how having higher pressures would change the need for good combustion chamber dynamics. though it might depend if youre going to run run a million PSI on different fuel than the pump stuff eg E85 or racegas as some people seem to slow combustion down to help things survive.

Paul Burke is a cluey guy and does alot of 2V BMW FI engines and his FI M20 pistons have the dome on them, having said that heaps of people of run any old dome shape without issue and seems to work ok.

The same with NA engines many run a flat top and they still run but do seem to prone to det atleast a few guys i know here have that issue.

[ForcedFirebird]

People who know more than me say that 1.5-2mm is the region to avoid as it is the worst for causing possibility of detonation/knock so if you're going to do it aim for 1.0mm (at most 1.25mm ). If the compression is low, fuel is good, the engine tuned well you can still run it but it's not ideal.

.9mm-1.1mm is the sweet spot. Basically rods will stretch at high RPM, much closer than that and we see rod bearing damage. The closer the piston is to the head at peak RPM the better - kinda like taking the head gasket out of the equation, so to speak.

[digger]

ive heard people run them down to 0.75mm with good rods and not too many rpm but on a street car there diminishing returns plus you want some allowance for a head skim down the track